1. Field of the Invention
The present invention relates to an optical communication system for transmitting a wavelength-division-multiplexed optical signal comprising a plurality of multiplexed optical signals having respective different wavelengths according to the wavelength division multiplexing technology, and more particularly to an optical communication apparatus, an optical communication system, and an optical transport method for an OADM (Optical Add-Drop Multiplexing) node which adds and drops an optical signal having a desired wavelength to and from a wavelength-division-multiplexed optical signal comprising a plurality of multiplexed optical signals having respective different wavelengths, and passes such a wavelength-division-multiplexed optical signal, and an OXC (Optical cross Connect) node which adds and drops an optical signal having a desired wavelength to and from a wavelength-division-multiplexed optical signal comprising a plurality of multiplexed optical signals having respective different wavelengths, and switches between paths for outputting optical signals having respective different wavelengths.
2. Description of the Related Art
<Wavelength Division Multiplexing Optical Communication System>
Optical communication systems based on the WDM (Wavelength Division Multiplexing) technology have been in growing usage in order to construct a backbone network which is capable of accommodating vast data traffic, typified by the Internet.
Such an optical communication system comprises a transmitting terminal station, repeating stations, a receiving terminal station, and an optical fiber transport path interconnecting those stations. The transmitting terminal station multiplexes a plurality of optical signals in respective channels into a wavelength-division-multiplexed signal and outputs the wavelength-division-multiplexed signal to the optical fiber transport path. The repeating station is positioned on the optical fiber transport path and amplifies the wavelength-division-multiplexed signal and compensates for chromatic dispersions. The receiving terminal station demultiplexes the wavelength-division-multiplexed signal that is input from the optical fiber transport path via the repeating station into a plurality of optical signals in respective channels, and receives those optical signals.
One conventional optical communication system is illustrated in FIG. 1 of the accompanying drawings. In FIG. 1, a transmitting terminal station comprises a plurality of transmitters (Tx) 2601 through 260N, multiplexer (MUX) 240, and optical amplifier 210. A receiving terminal station comprises a plurality of receivers (Rx) 2701 through 270N, demultiplexer (DMUX) 250, and optical amplifier 230. Optical amplifier 220 functions as a repeating station. Optical amplifiers 210, 220, 230 are combined respectively with DCMs (Dispersion Compensating Modules) 211, 221, 231 for compensating for a chromatic dispersion caused by an optical fiber transport path.
Though only one repeating station is illustrated in FIG. 1 for the sake of brevity, the optical communication system actually has a plurality of repeating stations depending on the length of the optical fiber transport path.
In the transmitting terminal station, transmitters (Tx) 2601 through 260N generate optical signals having respective wavelengths λ1, λ2, . . . , λN, and multiplexer 240 multiplexes the optical signals into a wavelength-division-multiplexed signal. Optical amplifier 210 amplifies the wavelength-division-multiplexed signal and outputs the amplified wavelength-division-multiplexed signal to the optical fiber transport path. When the wavelength-division-multiplexed signal is amplified by optical amplifier 210, the wavelength-division-multiplexed signal may be compensated for a chromatic dispersion.
The wavelength-division-multiplexed signal output to the optical fiber transport path is transmitted to optical amplifier 220 as the repeating station. After the wavelength-division-multiplexed signal is amplified by optical amplifier 220, the wavelength-division-multiplexed signal is transmitted over the optical fiber transport path to the receiving terminal station. When the wavelength-division-multiplexed signal is amplified by optical amplifier 220, the wavelength-division-multiplexed signal is compensated by DCM 221 for a chromatic dispersion which has occurred in the optical fiber transport path between the transmitting terminal station and the repeating station. In the receiving terminal station, the wavelength-division-multiplexed signal transmitted from the repeating station over the optical fiber transport path is amplified by optical amplifier 230, and then demultiplexed by demultiplexer 250 into optical signals having respective wavelengths λ1, λ2, . . . , λN, which are output respectively to receivers 2701 through 270N.
<Dispersion Compensation>
Dispersion compensation that is performed by the above optical communication system will be described in detail below.
The optical fiber transport path has such properties that it causes chromatic dispersion (hereinafter also referred to as “dispersion”) which tends to be applied to a signal that is passing through the optical fiber transport path. When the signal suffers accumulated dispersion, different frequency components of the signal are delayed by different amount, resulting in a serious signal waveform distortion. If the accumulated dispersion is too large, then the signal cannot properly received by the receiving terminal station.
Therefore, it is necessary to perform dispersion compensation by canceling a chromatic dispersion caused by the optical fiber transport path with a dispersion having a reverse sign. Different signs of dispersion are differentiated by “abnormal dispersion”, and “normal dispersion”. It is often to indicate abnormal dispersion with a sign of “+” and normal dispersion with a sign of “−”. These signs will be used in this specification. However, care should be taken because the signs may be used the other way around according to some standards. Dispersion has a unit of “ps/nm”.
Dispersion in an optical fiber transport path is compensated for by a dispersion compensating module (DCM). DCMs that are generally used today are in the form of a dispersion compensating fiber.
It is necessary to pay attention to two different ways of expressing a value of dispersion. According to one practice, the capability of a DCM is expressed as “DCM of −500 ps/nm”, for example. The phrase “−500 ps/nm” represents that the DCM is capable applying a dispersion of −500 ps/nm to a signal. According to the other practice, a signal with a dispersion of +500 ps/nm applied thereto is referred to as a signal having dispersion of +500 ps/nm. This expression is an idiomatic expression and should more accurately mean “a signal suffering an accumulated dispersion of +500 ps/nm”. A value of dispersion that a signal is suffering represents an accumulated value of dispersion that a medium has caused which the signal has been propagated through from the time when the signal started to be transmitted and the accumulation was nil.
<OADM Node>
Initially, optical communication systems were introduced because they are capable of transmitting optical signals over long distances. First, two components of a communication system, i.e., links and nodes, were constructed as optical devices. Thereafter, as the transmission capacity per link greatly increased according to the wavelength division multiplexing technology, the cost of a process of converting all wavelength-division-multiplexed signals into electric signals at an input section of each node, editing the electric signals, and then generating and outputting all wavelength-division-multiplexed signals again become burdensome. There was a demand for processing optical signals as they are in each node.
OADM (Optical Add-Drop Multiplexing) nodes were put to practical use. An OADM node is a node having functions to extract and receive certain wavelength channels from a wavelength-division-multiplexed optical signal and also to add and transmit certain wavelength channels to a wavelength-division-multiplexed optical signal. The OADM node allows wavelength channels passing therethrough to be processed at a greatly reduced cost, and makes the optical communication system economical.
A conventional optical communication system with an ODAM node is shown in FIG. 2 of the accompanying drawings. The optical communication system shown in FIG. 2 is similar to the optical communication system shown in FIG. 1 except that it has ODAM node 300 instead of the repeating station.
The OADM node is supplied with a single wavelength-division-multiplexed optical signal having a plurality of multiplexed wavelength channels, drops certain wavelength channels from the wavelength-division-multiplexed optical signal, adds certain wavelength channels to the wavelength-division-multiplexed optical signal, and outputs a single wavelength-division-multiplexed optical signal.
As shown in FIG. 2, OADM node 300 drops wavelength channels having wavelengths λ1, λ2, λ3 from a wavelength-division-multiplexed optical signal transmitted through an optical fiber transport path, outputs the dropped wavelength channels to receiver 70, and adds wavelength channels having wavelengths λ2, λ3 from transmitter 80 to the wavelength-division-multiplexed optical signal. Actually, the optical communication system has a plurality of receivers which are all denoted by 70 and a plurality of transmitters which are all denoted by 80.
<Conventional OADM Node Arrangements>
Heretofore, there are available several arrangements for use as OADM node 300 shown in FIG. 2. Such conventional arrangements for use as OADM node 300 will be described below.
FIG. 3 of the accompanying drawings shows a parallel OADM node for demultiplexing a supplied wavelength-division-multiplexed optical signal into a plurality of optical signals, processing the optical signals, and then multiplexing the processed optical signals into a wavelength-division-multiplexed optical signal. The OADM node comprises receiving amplifier 10, transmitting amplifier 20, demultiplexer 30, multiplexer 40, receiver 70, and transmitter 80. Receiving amplifier 10 is combined with receiving dispersion compensating module (receiving DCM) 11, and transmitting amplifier 20 is combined with transmitting dispersion compensating module (transmitting DCM) 21.
Receiving DCM 11 compensates for a dispersion of the wavelength-division-multiplexed optical signal supplied to receiving amplifier 10, and transmitting DCM 21 compensates for a dispersion of the wavelength-division-multiplexed optical signal output from transmitting amplifier 20.
DMUX 30 demultiplexes the wavelength-division-multiplexed optical signal from receiving amplifier 10 into optical signals having respective wavelengths λ1, λ2, λ3, . . . , λN. In FIG. 3, optical signals having respective wavelengths λ1, λ2, λ3 are dropped from DMUX 30, and optical signals having respective wavelengths λ4 through λN are output as through signals from DMUX 30 to MUX 40. The dropped optical signals having respective wavelengths λ1, λ2, λ3 are output to receiver 70.
Optical signals having respective wavelengths λ2, λ3 are added from transmitter 80 to MUX 40.
MUX 40 multiplexes the optical signals having respective wavelengths λ4 through λN from DMUX 30 and the optical signals having respective wavelengths λ2, λ3 from transmitter 80 into a wavelength-division-multiplexed optical signal, and outputs the wavelength-division-multiplexed optical signal to transmitting amplifier 20.
The arrangement shown in FIG. 3 is identical to a pair of multipliers and demultiplexer, connected back to back, each used in a terminal such as a receiving terminal station or a transmitting terminal station, and can share parts with terminals. MUX 30 and DMUX 40 may employ AWGs (Arrayed Waveguide Gratings) that have widely been used in the art and are inexpensive to manufacture. The OADM node shown in FIG. 3 is advantageous in that the through signals are not adversely affected when add/drop settings are changed.
FIG. 4 of the accompanying drawings shows a series OADM node which employs optical filters 141, 142 for acting on wavelengths that are to be added and dropped.
Optical filter 141 separates an optical signal having wavelength λ1 from a wavelength-division-multiplexed optical signal, and optical filter 142 separates an optical signal having wavelength λ2 from a wavelength-division-multiplexed optical signal and combines an optical signal having wavelength λ2 with the wavelength-division-multiplexed optical signal.
The arrangement shown in FIG. 4 is advantageous in that it is economical if the ratio of the number of wavelength channels that are added and dropped to the number of all wavelength channels is small. However, the arrangement shown in FIG. 4 is disadvantageous in that when add/drop settings are changed, the optical filters need to be taken into and out of working positions, and instantaneously interrupt all the through signals. Therefore, the arrangement shown in FIG. 4 is not suitable for use in OADM applications for dynamically changing add/drop settings.
FIG. 5 of the accompanying drawings shows a new device which has been introduced into the art in recent years, the device employing a wavelength blocker (WB).
The WB is a device capable of passing channels of certain wavelengths as through signals, of a supplied wavelength-division-multiplexed optical signal. Specifically, a WB manufactured by JDS Uniphase Corporation may be used.
The conventional OADM node shown in FIG. 5 has dividing coupler 51, wavelength blocker (WB) 110, and combining coupler 50 which are connected in series between receiving amplifier 10 and transmitting amplifier 20. DMUX 30 is connected to dividing coupler 51, and MUX 40 is connected to combining coupler 50. Dividing coupler 51 divides a wavelength-division-multiplexed optical signal output from receiving amplifier 10 and outputs the divided wavelength-division-multiplexed optical signal to DMUX 30, and combining coupler 50 combines an optical signal from WB 110 and an optical signal from MUX 40 with each other, and outputs a combined optical signal to transmitting amplifier 20.
WB 110 is a reconfigurable optical filter for selectively passing and blocking wavelength channels, and does not adversely affect through signals when it is reconfigured. WB 110 passes only wavelength channels as through signals of a wavelength-division-multiplexed optical signal after its dispersion has been adjusted by receiving dispersion compensating module 11. In FIG. 5, WB 110 is configured to block optical signals having respective wavelengths λ1, λ2, λ3 against passing therethrough.
With the conventional OADM node shown in FIG. 5, optical signals having respective wavelengths λ1, λ2, λ3 are demultiplexed from the wavelength-division-multiplexed optical signal by DMUX 30, and output to receiver 70. However, optical signals having respective wavelengths λ4 through λN pass as through signals through WB 110, and are output to transmitting amplifier 20.
The arrangement of the OADM node shown in FIG. 5 is similar to the parallel OADM node shown in FIG. 3, except through signals are differently handled. In the parallel OADM node, all signals including dropped signals and through signals are demultiplexed. In the OADM node having the WB, through signals having respective wavelengths λ4 through λN are not demultiplexed. Therefore, even if OADM nodes with WBs are connected in multiple stages, the through signals are prevented from being degraded by a narrower spectral range when the optical signal passes through multiple optical fibers of DMUXs 30.
FIG. 6 of the accompanying drawings shows an OADM node which is basically the same as the OADM node shown in FIG. 3, except that optical 2×2 switches 130 are added for automatic configuration. With the OADM node shown in FIG. 3, add/drop/through settings are made entirely through optical patch code connections that are manually set up to connect DMUX 30 and MUX 40. Though the optical patch code connections are best from the standpoint of structural costs, they are costly in terms of man-hours, tend to suffer setting errors if the number of wavelengths handled increases, and are not freely capable of selecting any desired wavelengths to be added and dropped. The OADM node shown in FIG. 6, the 2×2 switches 130 are associated with the respective wavelengths for automatically and remotely setting at least whether optical signals in the respective channels are to pass through or not.
<Optimum Value of Accumulated Dispersion of Received Signal>
In each of the above OADM nodes, receiving DCM 11 and transmitting DCM 21 are used to compensate for chromatic dispersion caused by an optical fiber transport path to reduce a value of accumulated dispersion. Generally, each node is configured to compensate for accumulated dispersion caused by an optical fiber transport path to eliminate the accumulated dispersion. Therefore, when a transmitted optical signal is received by the receiver, the received optical signal is supported to have no accumulated dispersion. However, it is known in the art that when a transmitted optical signal is received by the receiver, an optimum value of accumulated dispersion of the signal is often not zero. The optimum value of accumulated dispersion unit a minimum bit error rate. Consequently, the optimum value of accumulated dispersion represents a value of accumulated dispersion which makes the bit error rate at the receiver minimum. If complete dispersion compensation is performed to eliminate accumulated dispersion, then the waveform of the received optical signal should be identical to the waveform of the transmitted optical signal. However, the bit error rate is further improved by imparting dispersion. This phenomenon occurs because of phase modulation accompanied by intensity modulation that is mainly called “chirp”. In the presence of chirp, optical pulses are compressed by dispersion accumulation, improving the receiver sensitivity.
An optimum value of accumulated dispersion which is not zero is disclosed in Japanese patent No. 3337980, for example. FIGS. 10 and 11 of Japanese patent No. 3337980 indicate that there are cases wherein the penalty of the receiver sensitivity is lower when the accumulated dispersion is not zero(optimum) than when the accumulated dispersion is zero. In particular, FIG. 11 of Japanese patent No. 3337980 shows that accumulated dispersion is present across zero, the optimum value of accumulated dispersion is not zero in most cases, and the optimum value of accumulated dispersion changes depending on how chirp is given in the modulator.
Chirp which gives rise to the above phenomenon is a kind of phase modulation. Types of phase modulation include the chirp which is given by the nature of the transmitter from the time when the optical signal is transmitted, and also nonlinear phase modulation which is given by a optical nonlinear effect in the optical fiber transport path. The optical nonlinear effect refers to a phenomenon in which the refractive index of an optical fiber changes depending on the instantaneous optical power. The optical signal transmitted through the optical fiber is subject to nonlinear phase modulation due to the optical nonlinear effect. Optical nonlinear effects in a wavelength division multiplex system include SPM (Self Phase Modulation) caused in own channels and XPM (Cross Phase Modulation) caused by other propagating channels in parallel. Because these optical nonlinear effects take place when an optical signal enters from an optical amplifier into an optical fiber, an accumulated amount of nonlinear phase modulation differs depending on the launched power into the optical fiber and the number of repeating optical amplifiers, etc.
The above phenomenon is also revealed in Japanese laid-open patent publication No. 2003-318825. FIG. 2 of Japanese laid-open patent publication No. 2003-318825 shows an optimum value of accumulated dispersion represented by a line indicated as optimum. FIG. 2 shows that there are cases wherein the optimum value of accumulated dispersion is not zero, the optimum value of accumulated dispersion changes with the transmission distance, and the optimum value of accumulated dispersion also changes depending on the transmission process.
FIG. 7 of the accompanying drawings shows measured data of a bit error rate with respect to accumulated dispersion when an optical signal is received by a receiving terminal of a wavelength division multiplexing optical communication system. According to the measured data shown in FIG. 7, the error code ratio is best, i.e., minimum, when accumulated dispersion has a value of about +300 ps/nm. Therefore, the optimum value of accumulated dispersion is +300 ps/nm.
As the accumulated dispersion, i.e., the absolute value thereof, increases, the bit error rate becomes worse, and hence the optimum value of accumulated dispersion is not too large. Generally, the optimum value of accumulated dispersion falls in a certain range across 0 ps/nm.
For example, if the transmission rate is 10 Gbps and the code format is NRZ (Non-Return to Zero), then the optimum value of accumulated dispersion at the time the optical signal is received falls in the following range:|Dispersion range containing optimum value of accumulated dispersion|≦1000 [ps/nm]
If the code format is RZ (Return to Zero), then the dispersion range becomes about half, as indicated below:|Dispersion range containing optimum value of accumulated dispersion|≦500 [ps/nm]
As described above, an optimum value of accumulated dispersion of the signal when it is received is often not zero. The value of accumulated dispersion of the signal when it is received can easily be tuned to an optimum value in a simple receiving terminal station, but cannot in an OADM station because of structural limitations thereof.
Specifically, problems which occur with different values of dispersion at transmitting and receiving stations in a conventional system will be described below with reference to FIG. 8 of the accompanying drawings. It is assumed that the value of dispersion of a received optical signal is adjusted to +300 ps/nm. The value of dispersion can be adjusted using only receiving DCM 11. As a result, though the optical signal can be received with an optimum value of accumulated dispersion, through signals also suffer accumulated dispersion having a value of +300 ps/nm. On the other hand, transmitted signals, i.e., added signals, output from transmitter 80 have accumulated dispersion having a value of 0 ps/nm. When the through signals and the added signals are multiplexed by MUX 40, a wavelength-division-multiplexed optical signal output from MUX 40 includes a mixture of signals having accumulated dispersion of +300 ps/nm and signals having accumulated dispersion of 0 ps/nm. If signals having such different values of accumulated dispersion are produced in a plurality of OADM nodes, then it is difficult to manage the values of accumulated dispersion.
Consequently, it has heretofore been necessary to keep the accumulated dispersion nearly nil when the optical signal is received in the OADM nodes, because the dispersion of the transmitted signals and the dispersion of the through signals are same, so that the transmitted wavelength-division-multiplexed optical signal has a single value of dispersion. However, this practice faces a dilemma in that since the value of accumulated dispersion at the time the signal is received is not optimum, the transmitted signal cannot have its best performance unlike the data shown in FIG. 7, and a margin for dispersion variations is reduced.
<Conventional Dispersion Adjustment Process and Problems>
A straightforward solution to the above problem is provided by arrangements shown in FIGS. 9 and 10 of the accompanying drawings. In FIG. 9, individual auxiliary DCMs 60 are inserted for respective channels that have been demultiplexed. In FIG. 10, individual auxiliary DCMs 60 are inserted for respective channels that are to be multiplexed.
The arrangement shown in FIG. 9 is advantageous in that it offers a high level of adjustment freedom because the dispersion can be adjusted in each wavelength channel, and is put to practical use in a submarine transmission system which requires such adjustments. However, it is generally too costly to provide as many auxiliary DCMs 60 as the number of wavelengths involved, and the advantage of being capable of fine adjustment is not universal because it poses undue operational costs to optimize the fine adjustment capability to every application. Therefore, there is a demand for a solution without using individual auxiliary DCMs.
Another solution is disclosed in Japanese laid-open patent publication No. 2003-318825 referred to above. The disclosed solution employs an algorithm for seeking a combination of optimum values for holding accumulated dispersion within an allowable range for any wavelength path groups.
The algorithmic process is advantageous in that it requires no special hardware, but is disadvantageous in that as the number of wavelength paths increases, a process of finding a combination of optimum dispersion values compensation is practically infeasible, and it is difficult to handle a change in wavelength path settings because the addition of a new wavelength path needs the amount of dispersion compensation to be changed for the entire system.
DCMs that are generally used at present have a fixed amount of compensation. Therefore, if the amount of compensation is changed, then DCMs need to be replaced. Replacing a DCM requires a temporary circuit disconnection and results in an added cost due to the addition of a new DCM to replace the DCM. It is therefore desirable to keep the amount of dispersion compensation unchanged even if wavelength path settings are changed.
OXC nodes, which are an optical communication apparatus other than the OADM nodes, suffer the same problems as the OADM nodes described above because if accumulated dispersion of an optical signal when the optical signal is received is to be set to an optimum value, then an auxiliary DCM needs to be provided for each wavelength.